Development is organized in a series of short, fixed-length mini-projects called iterations
Iterations are also incremental Successive enlargement and refinement of a system Feedback and adaptation evolve the specification,
design and code How might iterative development be different from
prototyping? Output of each iteration need not be experimental
or a throw-away prototype Each iteration tries to be a production-grade subset
of final system
Le mieux est l’ennemi du bien
- Voltaire(The best is the enemy of the good.)
Why? Avoid “Paralysis by Analysis” – kills budget
without significant benefit Classic mistake: Too much time and money
wasted in the “fuzzy front end”
Each iteration involves choosing a small subset of requirements, and quickly designing, implementing and testing
Early feedback (from users, developers and tests) drives development
Requirements are capabilities and conditions to which the system and the project must conform
A prime challenge of requirements analysis is to find, communicate, and remember what is really needed, in the form that clearly speaks to the client and development team members.
A story of using a system fulfilling a goal◦ E.g., Deposit cash◦ A use case story consists of a set of alternative
scenarios
Actors are capable of active behavior ◦ E.g., Person, computer system, organization
Primary actors have goals that use case accomplish◦ E.g., Customer, Clerk
Supporting actors help use case accomplish goal◦ E.g., Bank, Database
See alistar.cockburn.us Use case name Scope Level (user-goal or subfunction) Actors: Primary, Secondary Stakeholders and interests (who cares about this use case,
and what do they want?) Preconditions (what must be true on start) Postconditions or Success guarantee (what must be true on
successful completion) Main success scenario (typical path, happy path) Extensions (alternate scenarios of success and failure) Special requirements (related non-functional requirements) Technology and data variations list (varying I/O methods) Frequency of occurrence Miscellaneous
Cockburn: Elementary Business Process (EBP) guideline:◦ “A task performed by one person in one place at one time,
in response to a business event, which adds measurable business value and leaves the data in a consistent state.”
Naively, can you apply the “boss test” for an EBP?◦ Boss: “What do you do all day?”◦ Me: “I logged in!”◦ Is Boss happy?
Size: An EBP-level use case usually is composed of several steps, not just one or two.
Which of following meets EBP & size guidelines?
◦ Negotiate a Supplier Contract◦ Rent Videos◦ Log In◦ Start Up
The others can also be modeled as use cases ◦ But focus first on essential cases (EBP level)
Keep use case names simple: Verb object◦ Deposit money. ◦ Not: Deposit money into checking. Why not?
Accomplish a user’s goal◦ Invalid PIN is not a use case. Why not?
Include Secondary Actors (e.g., Bank) Avoid ambiguity
◦ E.g., in the ATM problem, System could be the machine or the Bank’s back-end server
Start Up and Shut Down are use cases◦ Why do we usually not include them at first?
More use case guidelines
A use case diagram is not a flow chart Steps in the use case (such as enter PIN)
are not necessarily use cases. Why not?
Keep each step and alternative simple; e.g., don’t validate PIN and balance in same step (and same alternative scenario)
Transactions (such as deposit money and withdraw cash) are candidate use cases. Why?
UML has use case diagrams
Use cases are text, not diagrams
But a short time drawing a use case diagram provides a context for:◦ identifying use cases by name◦ creating a “context diagram”
Again, a use case diagram is not a flow chart!
Video Store Information System
Rent Videos
. . .Clerk
Show computer system actorswith an alternate notation tohuman actors.
primary actors onthe left
supporting actorson the right
Prefer use cases at the EBP level.
«actor»Credit
AuthorizationService
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Use cases describe functional requirements
Supplementary Specification (SS) captures non-functional reqs (URPS+):
Vision and Scope Features list Glossary (Data Dictionary) Business Rules Risk plan Iteration Plan
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Feature is a behavioral function a system can do
A feature is an externally visible service◦ E.g., system does investment rate of return◦ System does credit risk analysis
Why is a feature list useful when developing a Vision and Scope document?
Prefer short (10-12) feature list of most valuable benefits
Is honesty the best policy?
Contains a list of known and expected risks Business, technical, resource, and schedule
risks identified by probability and severity All significant risks should have a response
or mitigation plan
Rank requirements as: High (score high on all rankings; hard to add late) Medium (affects security domain) Low
by: Risk (includes both technical complexity and other factors, such as
uncertainty of effort and usability) Coverage (all major parts of the system are tackled in early
iterations) Criticality (refers to functions the client considers of high business
value)
Ranking is done before each iteration
Describes what to do in each iteration of product Usually first iteration implements core
functionality Need to consider risks and make estimates
◦ Eliminate biggest risk first◦ Worst risk is usually that the final product will
not meet the most important requirement◦ Estimate what can be accomplished in 2-3
weeks
There is a funnel of cost estimates◦ The earlier the estimate, the less accurate it is.
Inception, Analysis, Design, Construction, next phase, etc…
+/- 100% +/-50% +/- 25% +/-10%
Some inception estimates are +400/-75%
